Serendipitous formation of K₁₅NaSn₅F₃₆

1 Introduction

Fluorides are a versatile class of compounds, offering 0-dimensional to 3-dimensional structural motifs. The fundamental building blocks may consist of tetrahedral up to nonahedral units [1], [2]. Among these, the octahedral coordination sphere is one of the most commonly observed structural motifs [1], [3]. These entities can not only occur isolated, but also connected via common corners, edges, or faces [1], [3]. This can lead to the formation of quasi-isolated structural units, chains, or networks [1]. Most prominent representatives of the fluorides are the hexafluoridometallates A₂MF₆ (A=monovalent cation, M=tetravalent cation), A₃M′F₆ (M′=trivalent cation), and A′MF₆ (A′=divalent cation) [1], in which quasi-isolated octahedral MF₆ building blocks dominate the solid state structures. Recently, hexafluoridosilicates like K₂SiF₆ or Li₂SiF₆ exhibiting quasi-isolated octahedral units received a lot of attention as host materials for Mn⁴⁺. Ever since K₂SiF₆:Mn⁴⁺ was adopted as a red luminescent material in white light emitting diodes, numerous novel hexafluoridometallate phosphors have been discovered [4], [5], [6]. However, within the group of tin fluorides not only the hexafluoridostannates are known, but also compositions like ASn₂F₅ (A=monovalent cation), KSnF₃, A′SnF₄ (A′=divalent cation), A″SnF₇ (A″=trivalent cation), or Na₄Sn₃F₁₀ have been mentioned in the literature [7], [8], [9], [10], [11], [12], [13], [14], [15], [16], [17]. Nearly all of the published fluoridostannates contain tin in the oxidation state 2+. Therefore, these structures commonly exhibit a square pyramidal coordination of tin, which leaves space for the lone pair of electrons. The oxidation state of 4+ is only observed within the hexafluoridostannates like Cs₂SnF₆, KNaSnF₆, RbNaSnF₆, and CsNaSnF₆, and within A″SnF₇ (A″=trivalent cation) [11], [18], [19]. These materials containing Sn⁴⁺ show an octahedral coordination sphere around the tin atom. Interestingly, materials with large cell parameters containing solely isolated octahedral units are only mentioned scarcely. One example is (NH₄)₃SnF₇, which crystallizes in the cubic crystal system with the cell parameter a=1208.6(1) pm and a volume of 1.765 nm³ [20]. Here, we took the chance to characterize K₁₅NaSn₅F₃₆, which also exhibits an unusually large unit cell (hexagonal, a=1060.3(2), c=2011.9(4) pm, V=1.9588(7) nm³), whilst containing only simple but uniquely arranged building blocks.

2 Experimental section

3 Results and discussion

K₁₅NaSn₅F₃₆ crystallizes in the hexagonal crystal system with space group P6₃/m (no. 176). Its lattice parameters are 1060.3(2) and 2011.9(4) pm for a and c, respectively. The unit cell contains 114 atoms. This equals two formula units. Therefore, the Pearson code of K₁₅NaSn₅F₃₆ is hP114. The asymmetric unit of K₁₅NaSn₅F₃₆ consists of two tin, four potassium, one sodium, and seven fluorine positions. K1, K2, F1, F3, F5, F6, and F7 are located on the general Wyckoff positions 12i. Sn1, F2, and F4 are located at the special Wyckoff positions 6h, Sn2 and K4 (site occupancy factor (SOF)=0.5) at the special Wyckoff positions 4f, K3 at 4e, and Na1 is located at 2b. Therefore, the Wyckoff sequence of K₁₅NaSn₅F₃₆ is i⁷h³f²eb. Information about the crystal structure refinement is listed in Table 1. Further information about atomic coordinates, displacement parameters, as well as selected interatomic distances and angles are shown in Tables 2–5.

The main structural motifs of K₁₅NaSn₅F₃₆ are quasi-isolated [SnF₆]²⁻ and [NaF₆]⁵⁻ octahedra (Fig. 1). Within the two different [SnF₆]²⁻ octahedra the Sn‒F bond lengths vary from 195.1(2) to 198.7(3) pm (Table 4). This is in accordance with the value given for Cs₂SnF₆ in the literature (d_Sn‒F=195.2 pm) [25]. The bond angles confirm a distorted octahedral coordination sphere for both, Sn1 and Sn2 (Table 5). Sodium, in contrast, experiences a nearly perfect octahedral coordination sphere with Na‒F bond lengths of 226.3(2) pm and angles close to 90° (Table 5). The bond length is in accordance with the Na‒F bond length of 231.7 pm in NaF [26].

Fig. 1:

Main structural motifs of K₁₅NaSn₅F₃₆. The octahedral coordination spheres of Sn1, Sn2, and Na1 are shown on the left, center, and right, respectively. Bond lengths are given in pm.

These octahedral units are imbedded into a matrix of potassium cations. Within the structure, there are four crystallographically different potassium positions. The corresponding coordination spheres are depicted in Fig. 2. The K‒F bond lengths vary between 253.5(2) and 317.2(3) pm. It is noteworthy that the site occupancy of K4 is only 0.5. This is likely a result of a statistical disorder at this site. Around the K4 sites (Fig. 3, K4_A and K4_B created by symmetry operations), a coordination sphere is built up, which is surrounded solely by [SnF₆]²⁻ units (Fig. 3). In comparison, all other potassium cation positions are coordinated by a mixture of [SnF₆]²⁻ and [NaF₆]⁵⁻ units, which does not leave enough space for a disorder on these sites. Therefore, just one of the positions K4_A or K4_B is occupied, with a probability of 50% for each option.

Fig. 2:

Coordination spheres of K1 (top, left), K2 (top, right), K3 (bottom, left), and K4 (bottom, right). Bond lengths are given in pm.

Fig. 3:

Coordination sphere around the K4 position. Either K4_A or K4_B is occupied.

K₁₅NaSn₅F₃₆ exhibits a layer-like arrangement. Layer one (L1) consists of a mixture of quasi-isolated [SnF₆]²⁻ and [NaF₆]⁵⁻ octahedra (Fig. 4, marked blue), whereas layer two (L2) consists solely of quasi-isolated [SnF₆]²⁻ octahedra (Fig. 4, marked green). These layers are stacked along the crystallographic c axis (Fig. 4, top). Within every second layer of L1 the orientation of the octahedral units changes, like mirrored on (110). Additionally, every second layer of L2 is turned around the c axis by 180°.

Fig. 4:

Crystal structure projection of K₁₅NaSn₅F₃₆ along [100] (top, unit cell in solid black). Layers consisting of [SnF₆]²⁻ and [NaF₆]⁵⁻ units are marked in blue, layers consisting solely of [SnF₆]²⁻ are marked in green. Two examples of layers depicted along [001̅] are shown at the bottom.

The structural model was confirmed by bond-length/bond-strength (BLBS) [27], [28] and charge-distribution (CHARDI, program Chardi2015) [29], [30] calculations (Table 6). At the site K4, the CHARDI calculations predict a charge of +0.48. This is in accordance with the structural data, as the SOF of K4 is 0.5. In contrast, the site occupancy cannot be taken into account during the BLBS calculations. This leads to a calculated charge of +0.90 for K4.

4 Conclusion

K₁₅NaSn₅F₃₆ was characterized by means of single-crystal structure determination. It is, next to KNaSnF₆, the second representative in the class of potassium sodium fluoridostannates [19]. K₁₅NaSn₅F₃₆ exhibits its own structure type, showing quasi-isolated [SnF₆]²⁻ and [NaF₆]⁵⁻ entities that are imbedded into a matrix of potassium cations, which leads to an unusually large unit cell. One of the potassium positions shows a statistical disorder, because of a large void created by four [SnF₆]²⁻ octahedra. The structural model was confirmed by BLBS and CHARDI calculations. Crystals of K₁₅NaSn₅F₃₆ therefore have a large unit cell with an unusually high alkali metal to tin ratio of 3:1 (K:Sn). Additionally, this substance, if accessible as bulk material, might qualify as host material for doping with Mn⁴⁺ to yield a red phosphor, as it shows the preferable octahedral coordination sphere around the metal cations.